The present invention relates generally to the field liquid transport through narrow diameter tubing. More specifically, the present invention relates to the connection of narrow diameter external tubing to a port of a device.
Connection of narrow diameter external tubing to a fluid handling or analysis device is done using a port on the device. Common ports used in industry and research are known as a Chemical Process Industry (CPI) port, 10-32 CPI port or HPLC port. FIG. 1 shows a port 10 which are usually a threaded cavity into the device 12. The threaded cavity includes a threaded section 14, a conical section 16 and a bottom section 18, as shown in FIG. 1. The conical section 16 reduces the diameter of the threaded section 14 to that of the bottom section 18. The bottom section 18 is cylindrical in shape and has same diameter as the external tubing to be used in order to receive the external tubing. The bottom section 18 has a top 20 and a bottom 22. The top 20 of the bottom section 18 is the entrance to the bottom section 18. The bottom 22 of the bottom section 18 includes an opening 26 to the device 12 and a shoulder 24 about the opening 26. The purpose of the shoulder 24 at the bottom 22 of the bottom section 18 provides an interface between the end of the external tubing and the port. Current coupling devices to connect the external tubing to the port use a threaded member which drives a cone shaped ferrule into the conical section 16 of the port to provide clamping and sealing forces about the external tubing, typical of a compression type fitting. There are coupling devices which combine the threaded member and the cone shaped ferrule into single unit.
The current coupling devices are limited with regard to the maximum operating pressure which can be attained using the coupling device. Additionally, current coupling devices are limited with regard to eliminating parasitic effect referred to as ‘dead volume’ of a connection. Dead volume refers to any deviation from a cylindrical fluid path from the external tubing into the port. Dead volume occurs when the end of the external tubing to be connected to the port is not secured completely within the port. The effect of dead volume can also be present when the external tubing is deformed due to over tightening of compression fittings or improper cutting of the end of the external tubing. The current coupling devices are limited by the ability of the coned ferrule to hold the external tubing by means of friction applied by compression of the coned ferrule within the port. Even when the coupling device is properly installed using carefully prepared tubing, a negative drawback of the port design itself is that the design allows radial migration of fluids at the interface between the end of the external tubing, and the shoulder 24 of the bottom 22 of the bottom section 18 of the port.
Another aspect when using current coupling devices is the practice of using external tubing of smaller diameter than the bottom section 18 of the port. The bottom section 18 of the port is usually about one sixteenth of an inch in diameter. External tubing of one thirty-second inch diameter is commonly used with special ferrules, whereby a cylindrical section and a cone shaped section are combined end-to-end in a unified compression fitting and diameter adaptor. The cylindrical section forms a sleeve, which adapts the diameter of the external tubing to the diameter of the port. A shortcoming of this approach is the cone shaped section compresses, while the cylindrical shaped adaptor does not. This causes a void volume about the perimeter of the external tubing and within the perimeter of the cylindrical shaped section of the special ferrule. Furthermore, this type of ferrule is supplied with a fixed length cylindrical section, hence it is unable to adapt to ports with a deeper than average bottom section 18.
A final aspect of current coupling devices lies in the mechanism of failure. When pressures of fluids which are being transported through the port manifest themselves, a component of the pressure exerts a longitudinal force along the axis of the external tubing being clamped by the compression ferrule. The longitudinal force is countered solely by frictional force about the inside of the ferrule, where the ferrule contacts the external tubing. As fluids within the external tubing and volumes are pressurized and de-pressurized, a phenomenon know as ‘creep’ can occur. Creep causes an increasing dead volume to form between the end of the external tubing and the bottom section the port. Eventually, the external tubing will be pushed away from the bottom section and through the cone shaped ferrule, causing it to leak severely. This is commonly referred to as ‘tubing blow-out’.
It is an object of the present invention to provide a coupling between narrow diameter tubing and a port of a fluid handling device in such a way as to allow high pressure levels to be attained without fluid leakage.
It is an object of the present invention to eliminate dead volume at the interface between narrow diameter tubing and the interior of a port.